In a conventional factory or distribution warehouse, it is desirable to move loads along a transporting path that is predominately horizontal, but which may also involve travel uphill, downhill, diversion between subpaths, and the like. These systems provide an overhead conveyor system with rotating drive shafts, a carriage that is supported by the drive shaft or by a fixed support rail, and a carriage that has skewed drive wheels to engage with the rotating drive shaft. The engagement of the skewed wheels with the rotating drive shaft propels the carriage along the rotating drive shaft or along a fixed rail.
The present slip tube system relates to overhead conveyors of the type disclosed in U.S. Pat. No. 5,806,655 issued Sep. 15, 1998 to Tabler, in U.S. Pat. No. 5,785,168 issued Jul. 28, 1998 to Beall, Jr., in U.S. Pat. No. 4,203,511 issued May 20, 1980 to Uhing, in U.S. Pat. No. 3,164,104 issued Jan. 5, 1965 to Hunt, and in U.S. Pat. No. 3,850,280 issued Nov. 26, 1974 to Ohrnell. Shaft driven overhead conveyors have many advantages over the heavier load type conveyors such as the power and free conveyor; such advantages including quietness, cleanliness, less repair, easy diversion of load carrying carriages, buffering, speed variation along the conveying path, and generally greater flexibility in design.
Conventional prior art rotating shaft driven overhead conveyors may be limited in the amount of weight they may carry, in a slope of the incline/decline they may traverse, or in combinations of weight and the incline or decline. When in these situations, the carriage can undergo an uncontrolled slippage between the drive shaft and the driven wheels. For steep inclines and over heavy loads, the load cannot be propelled along the conveyor. For steep declines, the load can slide in an uncontrolled manner down the decline.
The present slip tube system addresses issues associated with prior art overhead conveyers having fixed axis, rotatable drive shafts engaging skewed driven wheels of a carriage to provide the carriage conveying force, wherein the carriage is supported by the drive shaft and/or a fixed support rail. This well known slippage problem of the rotatable drive shaft type of overhead conveyor has been partially solved by sand-blasting and then anodizing aluminum drive shafts to increase the friction therebetween, which adds expensive processing to the manufacturing. Though this is an improvement for some applications, in many cases, it is not enough. Moreover, sand blasting is hard on the drive wheels and the sand blasted surface loads up with dirt over time, and becomes less effective. Another solution that has been proposed is the over-coating of the drive tube with a high-friction material such as urethane. However, such coatings are expensive and can lift or peel. Also, once a tube has been sandblasted or over-coated, it is not readily available to be used on another part of the same line or another line where the higher friction may not be necessary or even undesirable. In addition to inclines and to a lesser extent, declines, the problem arises in other circumstances, for example: when a carriage with spaced apart trolleys for a single load (two trolleys being used to carry a greater load than can be carried with a single trolley) passes through a switch. Switches re-route a carriage traveling on one line to travel on another line. A trolley passing through the switch may not be powered, so that the rear trolley is the sole drive into the switch, and the front trolley is the sole drive out of the switch. In such a situation, the driving power is cut in half through the switch and slippage is more likely to occur, for example when the load is particularly heavy in the high load overhead conveyor of U.S. Pat. No. 5,785,168 issued Jul. 28, 1998, whose disclosure is incorporated herein in its entirety, by reference. Other solutions to the slippage problem can include drive ridges to increase traction such as that taught by Tabler in U.S. Pat. No. 7,178,661.
Alternately, there can be times when it is desired to have slippage (less friction) between the carriage drive wheels and the drive shaft. These instances can occur when a carriage containing a load is being driven by a rotating shaft, and encounters a stop. Stops can be found in an assembly environment to provide time for workers to complete an assembly task on one item before sending the completed item onto another work station via the overhead conveyer, or to accumulate a stack of items to be released one at a time in timed manufacturing. Once the stop is released, it is also desirable for the slipping system to re-engage with the rotating drive shaft to propel the carriage away from the stop.
Consequently, a significant need exists to be able to change the drive friction between the drive shaft and driven rollers of the rotatable drive shaft type of overhead conveyor systems, to make the drive friction changes work with conventional rotatable drive shafts without modification to the drive shafts, to make the drive friction changes easy to install or remove on existing drive components, and to provide more or less motive/drive force on the carriage as required.